“Drug resistance” means a circumstance when a disease (e.g., cancer) does not respond to a therapeutic agent. Drug resistance can be intrinsic, which means that the disease has never been responsive to the therapeutic agent, or acquired, which means that the disease ceases responding to the agent or agents to which the disease had previously been responsive. “Multidrug resistance” is a type of drug resistance wherein a disease is resistant to a variety of drugs that can be functionally unrelated, structurally unrelated, or both. Multidrug resistance is a problem associated with cancer and other conditions, such as bacterial, viral, protozoal, and fungal diseases.
One cause of multidrug resistance in cancer patients is that many cancer cells express high levels of the transmembrane transport proteins, such as Pleiotropic-glycoprotein (also known as Pgp, P-glycoprotein, gp-170, or MDR1) and MRP1 (see Borst, P., “Multidrug resistance: A solvable problem?” Annals of Oncology, 10, suppl. 4, pp. S162-S164 (1999)). In adenosine-triphosphate driven processes, these transport proteins export hydrophobic compounds (such as vinblastine, daunorubicin, doxorubicin, etoposide, vincristine, and TAXOL®, which are cytotoxic drugs useful for treating cancer) from the cell in an effort to protect the cell from harm. The transport proteins remove the compounds from the cell prior to their having a lethal effect on the cell (see Legrand, et. al, “Simultaneous Activity of MRP1 and Pgp Is Correlated With In Vitro Resistance to Daunorubicin and With In Vivo Resistance in Adult Acute Myeloid Leukemia”, Blood, Vol. 94, No. 3, pp. 1046-1056 (1999); and Zhu, B. T.; “A Novel Hypothesis for the Mechanism of Action of P-glycoprotein as a Multidrug Transporter,” Molecular Carcinogenesis 25, pp. 1-14 (1999)). Although it is not currently known which of these two classes of proteins is more important for multidrug resistance, and indeed it may be that the class (or classes) of protein which is important depends on the type of cancer and the particular drug or drugs used to treat the cancer, Pgp is known to be highly expressed in approximately 50% of human cancers which require drug therapy. Consequently, Pgp is believed to be a major cause of multidrug resistance.
Other types of multidrug resistance, such as antibacterial, antiviral, and antifungal multidrug resistance may also be caused by the action of transport proteins that are similar to Pgp, and others (see “Annual Reports on Medicinal Chemistry—33; Section III Cancer and Infectious Diseases” ed. Plattner, J., Academic Press, Ch. 12, pp. 121-130 (1998)).
Furthermore, Pgp is also expressed at high levels in the gastrointestinal tract, liver, kidneys, and brain, and therefore Pgp represents a major pharmacological barrier to the bioavailability of many drugs (see Amudkar, et. al in “Biochemical, Cellular, and Pharmacological Aspects of the Multidrug Transporter,” Annu. Rev. Pharmacol. Toxicol., 39, pp. 361-398 (1999)). For example, the oral bioavailability of many nutrients and drugs is negatively affected by the action of Pgp present in the gastrointestinal tract. “Oral bioavailability” means the ability of a drug or nutrient that is administered orally to be transported across the gastrointestinal tract and enter into the bloodstream. In addition, Pgp adversely affects penetration of many drugs through the blood-brain barrier.